/ride allows a mob to mount anything it can normally mount. But if you try making a skeleton mount a spider through /ride it said it can't mount the target. But skeletons can mount spiders.
Steps to reproduce:
1. spawn a skeleton with a spawn egg
2. do /ride @e[type=skeleton,c=1] summon_ride spider
Expected result:
The skeleton should get a spider mount.
Actual result:
It says the the ride was summoned but couldn't be added to skeleton.
Linked issues
duplicates 1
Comments 2
I have added your description to MCPE-97250.
There are two other reports on this however they do not explain how to do this. They also have no response and are from months ago.